Artist’s illustration of the extrasolar ring system circling the young giant planet or brown dwarf J1407b. The rings are shown eclipsing the young sun-like star J1407, as they would have appeared in early 2007. Image credit: Ron Miller

Feb 3, 2015
The best evidence of exomoons so far

Sen—New results describing the structure of a humongous ring system around a giant exoplanet or brown dwarf (dubbed J1407 b) were recently announced. These rings are the first ever to be discovered outside the Solar System, and they are massive. They are estimated to be about 200 times larger than Saturn’s, and the really exciting bit is that the architecture of the rings provides the first indirect evidence for the existence of 'exomoons'—moons outside of our Solar System.

The rings were discovered back in 2012 by Eric Mamajek at the University of Rochester in public data taken in 2007 from the SuperWASP project. SuperWASP is a UK-led ground-based survey using the transit method to detect exoplanets. SuperWASP is comprised of two observing platforms (one in the Northern Hemisphere in the Canary Islands and the other in the Southern Hemisphere in South Africa). The observing platforms are equipped with eight wide-field cameras, each capable of monitoring about 100,000 stars. These cameras stare night after night at a region of sky looking for the drop in light due to a exoplanet passing in front or transiting its host star.

In this case planet/brown dwarf J1407 b, thought to be bigger and heavier than Saturn and Jupiter, doesn't transit at all, but fortunately the rings are angled just enough to transit along the Earth's line of sight producing a complex series of dips in the light curve, the measurements of a star's brightness over time, of young Sun-like star J1407.

Mamajek and collaborators had been using the SuperWASP data for another project and happened to spot a strange dimming event in J1407's light curve that peaked their curiosity. The dimming event lasted for 56 days. This is puzzling because the majority of planet transits should typically last on the order of a few to tens of hours. Something occurring for tens of days implies a very big extended thing moving in front of the star.

Their best hypothesis was that they were seeing a disk of material around a body orbiting the star pass in front of J1407 blocking out a portion of its light. In the original discovery paper a simple model with at least 3 rings explains the bulk features of the drop in light observed, but there is more fine structure in the light curve that the model couldn't explain. Matthew Kenworthy from Liden Observatory in collaboration with Mamajek have now undertaken very detailed modeling to study the structure and extent of the ring system. Their model resolves a whopping 96 individual rings matching the bumps and wiggles visible in the SuperWASP data. Below is a fantastic animation of the SuperWASP light curve lined up with Kenworth's ring model.

In this new analysis Kenworthy and Mamajek find a gap in the rings at 0.4 AU (61 million km). This 4 million km wide gap could be easily carved out by an exomoon with a mass less than 0.8 times that of Earth with an orbital period of about 2 years. For a body of J1407 b's size, it's not so surprising for there to be moons orbiting inside and out of the rings.

In our Solar System Jupiter, Saturn, Uranus and Neptune all host a myriad of moons; Saturn's moon count is now at 62. For example in Saturn's rings, the F ring is bounded by moons Prometheus and Pandora on either side. The majority of these satellites are thought to have formed in place from the disk of icy material left over around the giant planets after they stopped accreting gas and solids from the solar nebula. Additionally, the gravitational effects of moons have been observed to direclty or indirectly create gaps in Saturn's ringsas well as keep the ring structure intact shepparding diffusing particles back into the ring.

The Cassini spacecraft gets up close and personal with Saturn's F ring and moons Prometheus (at right - interior to the ring ) and Pandora (at left - exterior to the ring). Image credit: NASA/JPL/Space Science Institute

The rings of J1407b span up to 0.6 AU (90 million km—that's more than half the distance between the Earth and Sun!). At these distances the outer rings are at distances far enough away that the gravitational influence of the J1407b is predicted to not be strong enough to tear apart any forming planetoid. That likely means that there are moons accreting right now out of the icy debris.

A similar process is thought to have occurred in Saturn's rings when they were much larger and more massive producing the Saturnian satellites. The Cassini spacecraft has been orbiting Saturn for nearly 10 years getting a close up of the ring structure, the dynamics of the ring particles, and the interactions between the rings and Saturn's moonsboth outside and embedded within the ring complex. In 2013, Cassini spotted a disturbance at the outer edge of Saturn's A ring. It has been suggested it might be caused by the gravitational tug of a newly formed moon migrating out of Saturn's rings. If true, this would be Saturn's 63rd moon and probably one of the last to be birthed from its depleting rings.

Disturbance in Saturn's A ring captured by the Cassini spacecraft possibly caused by the gravitational influence of an unseen moon that may have recently formed and be migrating out of Saturn's rings. Image credit: NASA/JPL-Caltech/Space Science Institute.

The gaps in the rings of J1407 b is our best evidence yet for exomoons to exist and also be actively forming. There are other active efforts to search for exomoons like The Hunt for Exomoons with Kepler (HEK), which uses data from NASA's Kepler mission, but nothing definitive has thus far been detected yet.

Exo-ring systems like the one orbiting J1407 could give astronomers snapshots into moon formation around gas giant planets.With new ground-based and space-based transit surveys coming online in the next decade, this is likely the first of many more ring/moon complexes to be found.